Use of suitable precursors in ALD processes is currently the topic of much research, where the selection of a proper precursor affects the purity and growth rate of a deposited layer. For example, Fluorine-containing precursors, though they have higher vapor pressures, are not desirable because strong ligand-M bonds result in low growth rates, and weak bonds in the ligand result in probable F contamination in the grown films. The most commonly used Sr precursor is the β-diketonate precursor Sr(tmhd)2 (tmhd=2,2,6,6-tetramethyl-3,5-heptanedione), also referred to as dipivaloylmethane (DPM). ALD of Sr with tmhd precursors often leads to films with significant carbon contamination or formation of the SrCO3 phase. Diketonates have stronger bonds to metal atoms than cyclopentadienyl rings and weaker bonds within the ligand, suggesting that diketonate precursors will have lower growth rates and more likely carbon contamination in the film. Literature suggests that the most thermally stable and volatile Ba precursors are Cp precursors with tert-Butyl and i-Propyl ligands. It has been reported that the vapor pressure stability of Ba(tmhd)2 is low at typical growth temperatures, and it has been found that Sr and Ba(tmhd)2 precursors decompose in the gas phase at substrate temperatures 300° C., whereas Sr or Ba atoms are incorporated into films at substrate temperatures of ≧400° C. Precursor thermal decomposition suggests that the precursor will not be suited to ALD, as self-limiting reaction cannot be achieved.
Accordingly, there is a need to develop a method of deposition of strontium oxide materials that reduces contamination yet enhances film growth rate.